1. Field of the Invention
The present invention is related to an automatic travel control method and apparatus for an unmanned vehicle, and particularly to an automatic travel control method and apparatus for keeping the body of the vehicle parallel with a specified guide path within the shortest possible distance at the end point of any curve in the guide path.
2. Description of the Prior Art
Conventionally, an unmanned vehicle system, which includes a front wheel and two rear wheels in the body thereof, has been developed. In the system, both steering and driving motors are equipped in the front wheel.
On the other hand, a guide wire (hereinafter, called a "guide path") is arranged in or on the floor of the factory and is energized by a low frequency alternating current, for example, between 3 kHz to 10 kHz, thereby generating a magnetic field therearound.
A pair of sensing coils, which sense the magnetic field generated by the guide path and which provide voltage outputs representative of the deviation of the coils from the guide path, are located at the left and right sides of the vehicle. The steering motor is then controlled based on the difference between the voltage outputs so that the front wheel can be turned to the left or right.
In the conventional unmanned vehicle system, the pair of sensing coils are symmetrically positioned at the left and right sides of the body of the vehicle. A pair of band-pass filters, each of which allows only a voltage signal with the same frequency as the alternating current flowing in the guide path to pass therein, are connected to the respective output terminals of the coils, and a pair of operational amplifiers are then connected to the respective output terminals of the band-pass filters. The two signals from the operational amplifiers are fed to a differential amplifier.
In the configuration described above, when the vehicle travels along a linear section of the guide path, voltage signals with a similar magnitude are induced to the sensing coils. Accordingly, the differential amplifier outputs a signal of 0V! to the steering motor, thereby not activating the steering motor. Consequently, the vehicle travels straight along the guide path.
On the other hand, when the vehicle travels along a curved section of the guide path, different voltages are induced to the sensing coils. The differential amplifier then amplifies the difference in voltage and supplies the amplified voltage signal to the steering motor. Thus, the steering motor is activated to cause the vehicle to turn left or right along the guide path.
FIG. 1 is a diagram showing the various positions of a traveling vehicle according to a conventional unmanned vehicle system.
Referring to FIG. 1, in the conventional unmanned vehicle system, the rear part of the vehicle 10 travels with a tendency toward the inside of the guide path 15 due to the difference in the rotational radius of the front and inner rear wheels.
Consequently, the body of vehicle 10 cannot remain parallel with a conveyor 20 located at the linear section adjacent to the curved section in the guide path 15, which causes errors in transferring and loading work.
On the other hand, an example of a unmanned vehicle system is disclosed in Japanese Patent Laid-Open Publication No. 297707 (1989).
However, the unmanned, vehicle system disclosed in the Japanese Publication is intended to guide an unmanned vehicle in a correct direction even where guide paths intersect by determining the correct travel path from the output of two sensing circuits which resonate with a guide frequency for the unmanned vehicle where the guide paths for a plurality of unmanned vehicles with different guide frequencies intersect.